Current step-down transformers with the primary winding connected in series with the load circuit, the current in which is to be measured, are an old measuring technique, particularly for ammeters and wattmeters connected across the secondary winding, but have also been used in more modern sense schemes for electronic applications. The circuit defined above is for instance known from "High-Frequency Switching Power Supplies: Theory and Design", G. Chryssis, McGraw-Hill Publishing Company, 1989, particularly p. 203 and 221. In the first, isolated current sensing has the secondary winding shunted by a diode in series with a resistance, the latter shunted on the input of a differential amplifier part of a power supply control circuit. In the second, the diode and resistance series combination part of a transistorized detector is again across the secondary with the primary winding of the current step-down transformer monitoring the load current of a power supply.
Such known techniques are however not suitable as such when energy consumption is at a premium and a fast reacting detector is needed as is the case for instance in recent high frequency electronic power supply circuits using power MOS transistors (MOSFETs) in integrated circuits. Using MOSFETs for the switches of such DC/DC converters but Schottky diodes for the output rectifiers has in recent years lost ground to the latter relying also on MOSFETs as synchronous rectifiers. Indeed, conduction losses in the rectifier unit of the converter can in this way be reduced substantially. Even though such losses are only a relatively low percentage of the output power, they represent an important fraction of the total converter losses and in many electronic applications maximum efficiency is very much in demand. Representative figures readily illustrate the comparison between a Schottky diode and a MOSFET as output rectifier when considering a 0.5 volt drop for the first and an ON resistance of 15 milliohms for the second. With a mean output current of 3 amperes for the converter, the conducting diode thus dissipates 1.5 watt while the MOSFET conduction loss will only be 0.185 watt widen assuming a form factor leading to an RMS (Root Mean Square) value of 3.5 amperes. Even though 1.5 watt is only 2.5% of the converter output power, if the output voltage is 20 volts it can constitute approximately 30% of the total converter losses. Thus, the very substantial conduction loss reduction afforded by MOSFETs as output recitifiers can be an attractive solution for many applications.
Representative considerations on such MOSFET use have been given by R. Blanchard and R. Severns in Section 5.6 on p. 5-69 to 5-86 of the "Mospower applications handbook", Siliconix Inc., 1984, i.e. MOSFETs move in on low voltage rectification (Siliconix Technical Article TA 84-2). Therein two main gate drive circuits for the MOSFET output rectifiers are discussed, i.e. based on use of auxiliary windings on the DC/DC converter output transformer secondary or independent gate drive circuits with proper timing, the latter dependent on the type of converter circuit. Additional considerations on such output MOSFET rectifier gate drive are also to be found for instance in the IEEE Power Electronics Specialists Conference Record, 1985, p. 355 to 361, "The design of a high efficiency, low voltage power supply using MOSFET synchronous rectification and cur rent mode control", by R. Blanchard and P. E. Thibodeau. In particular, the importance of the form factor of the converter output current is noted since different timing methods for gate-drive signals can lead to substantial differences in the RMS current as compared to the average, i.e. a 6% instead of 6.5% increase, which, as noted above, affects the MOSFET output rectifier efficiency far more than the Schottky diode.
In the above 1984 handbook it is pointed out that the additional complexities of using an independent gate drive are to be viewed in the light of various advantages some of which linked to the converter design or topology. In "An assessment of the use of resonant-mode topologies for high frequency power conversion", P. 331 to 337 of the Proceedings of the European Space Power Conference, Madrid, Oct. 2-6, 1989, S. H. Weinberg and C. D. Manning include ZVS (Zero-Voltage-Switched topologies) such as the Class-E resonant converter which is one of the circuits using an inductance and a capacitance both in series with the output transformer primary winding. While this is a single-ended DC/DC converter using only one MOSFET switch, the series circuit can also be fed through a full bridge using four switches. With each pair of diagonally opposite MOSFETs simultaneously turned on during their respective half-cycle when the voltage across them is close to zero, the reversal of the DC voltage applied to the series circuit including the primary winding takes place at a time of the half-cycle corresponding to a peak current value.
Unfortunately, as already noted in the above 1984 article in connection with the use of auxiliary transformer windings for the gate drive, not all circuit topologies are adapted to the desired way of controlling MOSFETs acting as output rectifiers, e.g. the quasi-square wave converter using a series output coil contrary to the buck or boost-derived converters, or the above full bridge ZVS converter, all three without a series inductance immediately preceding the output shunt capacitance.
When neither a clock synchronised command of the MOSFET output rectifiers nor the use of auxiliary transformer windings are suitable options for the MOSFET gate drive, as is the case for instance for the above full bridge ZVS converter, a suitable AC current detector would be desirable. Indeed, ideally, a rectifying MOSFET should be turned on as a positive current would tend to appear therein, turning off occurring when it would tend to become negative.